Microsoft has their own RAW viewer/thumbnailer...but you must have a Genuine install of the o/s.
...and from their website, this:
The RAW Image Thumbnailer and Viewer
consists of two components: a lightweight "shell extension" for Windows XP that provides high quality thumbnails in Windows Explorer, and a RAW Image Viewer application with an interface similar to the Windows Picture and Fax Viewer. (Please see the readme file or the companion whitepaper for complete details.)
Supported camera models and RAW image types:
Most current models of Canon and Nikon cameras are supported. Please see the companion whitepaper or the readme file for complete details.